Firestorm stores usernames and passwords in an encrypted credentials file inside your user settings folder. This file can only be used on one computer; it can't be decoded on a different computer.
If you don't want the viewer to store passwords:
Firestorm may complain that it cannot decode the credentials when you launch the viewer. This can be because there is actual corruption in the file, or because the file came from a different computer, or because of an operating system update. In some cases you can simply re-enter your credentials and the file will self-repair. If it warns you on each login, it will be necessary to delete the credentials file.
If you backup your settings, also delete the same file bin_conf.dat before attempting to use the restore option.